OlgaD wrote:Well... I think I know the meaning of those words but don't know the DIFFRENCE between them. I checked it in a dictionary...

I see, Olgad.
When you study Japanese, I don't think it's a good idea to analyse the subtle differences in the word variety.

今日、学校へ行った。（きょう、がっこうへいった）
There is no problem.
If you do want phrase it (or pronounce it) like "こんにち" , it would sound out-of -date today. But not totally wrong.
It seems to me that to know those diffences in definitions would be a waste of time for now.
There are tons of words and expressions out there.
You don't need to write a grammar book to speak a language.
That's my personal opinion.

こんにち is the 音読み of 今日(Chinese pronunciation), きょう is 訓読み(Japanese pronunciation). Generally speaking, 音読み sounds harder than 訓読み. The choosing between two reading methods are almost convention and custom.

I think many dictionaries don't quite kindly well explain the differences among words.
And I think it's good to ask differences here in this forum (but in many cases, there may not be quite obvious differences).

Obviously there are two different categories:
this present day = きょう (今日) and ほんじつ (本日)
this present time, period etc = こんにち (今日) and いまどき (今時)

And there are usage differences of the words in these two categories:
きょう common
ほんじつ formal
And
こんにち formal
いまどき common

It's alway easy to ask a question going "what's the difference?"
For instance, however, what do you make of it if someone asked you;
what's the difference between "it" and "that" and "which" ?
The questioner would be expected to explain what way does s/he have the question in.

That's what I wanted to point out.
I apologize if I sounded too blunt.